Tribology And Maintenance Engineering (ME-802 (B)) - Unit 4 Short Notes

UNIT 4: TRIBOLOGY AND MAINTENANCE ENGINEERING

I. FUNDAMENTALS OF TRIBOLOGY

• Definition & Scope: Tribology is the science and technology of friction, wear, and lubrication of interacting surfaces in relative motion.

• Historical Significance:

  • Term coined by Jost Report (1966, UK) highlighting massive economic losses (estimated 1-4% of GDP) due to inadequate tribological knowledge.

  • Ancient origins (lubrication in Egyptian pyramids, bearings in Roman chariots).

• Industrial Impact: Directly influences energy efficiency, reliability, maintenance costs, and product lifespan across all mechanical systems (automotive, aerospace, manufacturing, biomedical).

[!TIP] Exam Focus: Be prepared to state the Jost Report's significance and quantify tribology's economic impact.

II. CONTACT MECHANICS AND SURFACE INTERACTION

Types of Contact

Hertzian Contact Theory

• Assumptions:

  1. Both materials are homogeneous, isotropic, elastic (obeys Hooke's law).

  2. Surfaces are smooth, frictionless, and initially non-conforming.

  3. Deformations are small compared to dimensions.

  4. Each body is an elastic half-space (infinite in extent below the surface).

• Key Outputs: Calculates contact area (a or b), maximum contact pressure (p₀), and pressure distribution.

• For Sphere-on-Flat (or Sphere-in-Socket):

$$ a = \left( \frac{3FR}{4E^*} \right)^{1/3} $$

$$ p_0 = \frac{3F}{2\pi a^2} = \left( \frac{F E^{*2}}{4\pi R} \right)^{1/3} $$

Where:

*   $F$ = Normal load (N)

*   $R$ = Radius of sphere (m)

*   $$\displaystyle E^* $$ = **Effective Elastic Modulus**: $$\displaystyle \frac{1}{E^*} = \frac{1-\nu_1^2}{E_1} + \frac{1-\nu_2^2}{E_2} $$

*   $$\displaystyle E_1, E_2 $$ = Elastic moduli; $$\displaystyle \nu_1, \nu_2 $$ = Poisson's ratios.

[!TIP] Common Pitfall: Remember $$\displaystyle E^* $$ combines both materials. For identical materials, $$\displaystyle E^* = E/2(1-\nu^2) $$.

Adhesion and Stick-Slip

• Stick-Slip: Phenomenon where surfaces alternately stick (static friction) and slip (kinetic friction). Caused by friction-velocity characteristic (kinetic friction < static friction) and system elasticity.

• Implications: Causes vibrations, noise, surface damage, inaccurate motion (e.g., in machine tools, brakes, seismic events).

• Reducing Adhesive Friction:

  1. Use lubricants to separate surfaces.

  2. Apply surface coatings (low shear strength like PTFE, MoS₂).

  3. Surface texturing to trap lubricant or reduce real contact area.

  4. Select materials with low mutual solubility/adhesion.

III. FRICTION

Laws of Friction (Amonton's Laws)

1. First Law: Force of friction is directly proportional to normal load ($$\displaystyle F_f \propto N $$).

2. Second Law: Force of friction is independent of apparent contact area.

3. Third Law (often included): Kinetic friction is independent of sliding velocity (for most metals, at moderate speeds).

Exceptions to Classical Laws

• Very low loads (adhesion dominates, area matters).

• Very high speeds (friction increases).

• Very smooth/clean surfaces (adhesion increases).

• Non-metallic materials (rubber, polymers).

• Seizure at high loads/pressures.

Bowden and Tabor's Theory

• Real Area of Contact ($$\displaystyle A_r $$): Due to surface roughness, actual contact occurs at asperity junctions. $$\displaystyle A_r \propto \frac{F}{H} $$ (where $H$ = hardness).

• Friction ($$\displaystyle F_f $$): $$\displaystyle F_f = \tau \cdot A_r = \tau \cdot \frac{F}{H} = \mu F $$, where $\tau$ = shear strength of asperity junctions.

• Conclusion: $$\displaystyle \mu \approx \frac{\tau}{H} $$. Thus, friction depends on shear strength of junctions and hardness of softer material.

Factors Influencing Friction

IV. WEAR

Definition & Major Types

Factors Affecting Wear (Five Key Factors)

1. Material Pair (Tribological Pair): Hardness, ductility, compatibility.

2. Applied Load: Higher load → higher contact stress → accelerated wear.

3. Sliding Velocity: Affects temperature, lubrication, oxidation rate.

4. Environment: Presence of corrosive media, dust, moisture drastically changes wear mode.

5. Lubrication & Surface Finish: Adequate lubrication separates surfaces; smooth finish reduces abrasive/adhesive wear.

[!TIP] Exam Answer Structure: For "explain five factors," pick the above five and give one-line impact for each.

V. LUBRICATION THEORY

Lubrication Regimes (Based on $$\displaystyle \lambda = \frac{h}{R_q} $$)

• Boundary Lubrication ($$\displaystyle \lambda < 1 $$): Surfaces in close proximity. Friction & wear controlled by surface films ( adsorbed layers, chemically reacted layers). High friction, high wear.

• Mixed Lubrication ($$\displaystyle 1 < \lambda < 3 $$): Partial separation by lubricant. Some asperity contact. Transition regime.

• Hydrodynamic Lubrication ($$\displaystyle \lambda > 3 $$): Full fluid film separates surfaces. Friction due to viscous shear of fluid. Very low wear, friction depends on viscosity & speed.

Hydrodynamic Lubrication (HL)

• Principle: Wedge effect – converging gap between surfaces draws in lubricant, building hydrostatic pressure that supports load.

• Governing Equation: Reynolds Equation (for incompressible Newtonian fluid, steady state):

$$ \frac{\partial}{\partial x} \left( h^3 \frac{\partial p}{\partial x} \right) + \frac{\partial}{\partial y} \left( h^3 \frac{\partial p}{\partial y} \right) = 6\mu U \frac{\partial h}{\partial x} $$

Where $h$ = film thickness, $p$ = pressure, $\mu$ = viscosity, $U$ = velocity.

• Bearing Design: Minimum film thickness ($$\displaystyle h_{min} $$) must exceed combined surface roughness ($$\displaystyle R_q $$) to avoid asperity contact. Sommerfeld number is key dimensionless parameter for bearing analysis.

Elasto-Hydrodynamic Lubrication (EHL)

• Principle: Occurs in non-conforming contacts (rolling/sliding: gears, bearings). High contact pressures (GPa) cause elastic deformation of surfaces and viscosity increase of lubricant.

• Key Features:

  1. Hertzian pressure distribution modified by lubricant pressure.

  2. Exit constriction in film thickness due to viscosity-pressure effect.

  3. Very thin films (nanometers to micrometers) but still full separation.

• Applications: Rolling element bearings, gears, cam-followers.

[!TIP] Distinguish HL vs EHL: HL: Rigid surfaces, low pressure, thick films. EHL: Elastic deformation, high pressure, pressure-viscosity effect critical, thin films.

VI. BEARINGS

Classification

Cylindrical Roller Bearings (CRB)

• Construction: Cylindrical rollers guided by inner/outer rings. No cage (full complement) or with cage.

• Operation: Line contact (vs point contact in ball bearings). High radial load capacity.

• Advantages:

  • High radial load capacity and rigidity.

  • Low friction at high speeds (with good lubrication).

  • Can accommodate high-speed rotation.

• Disadvantages:

  • Sensitive to misalignment (angular misalignment causes edge stresses).

  • Limited axial load capacity (unless with flanges).

  • Speed limit lower than ball bearings due to roller centrifugal forces and skidding.

VII. SURFACE ENGINEERING AND COATINGS

Coating Techniques

• Physical Vapour Deposition (PVD):

  • Process: Vacuum environment. Physical process (evaporation/ sputtering) of coating material (TiN, TiAlN, DLC) onto substrate. Atoms travel in line-of-sight.

  • Sketch:

    DiagramCANVAS: Show vacuum chamber, substrate (cathode), target (anode), plasma glow, coating deposition on substrate. Arrows show sputtered atoms.

  • Characteristics: Thin (1-5 µm), hard, adherent, good for cutting tools, molds.

• Electroplating:

  • Process: Substrate as cathode in electrolyte containing metal ions (e.g., Cr, Ni, Cd). Current reduces ions, forming metallic coating.

  • Advantages: Uniform coating on complex shapes, low cost, can build up thickness.

  • Disadvantages: Hydrogen embrittlement (especially Cr), toxic waste (cyanide, chromium), poor adhesion on some substrates, coating is tensile.

• Hard Facing (Weld Overlay):

  • Process: Welding process (manual/semiautomatic) depositing hard, wear-resistant alloy (Stellite, carbide-based) onto base metal surface.

  • Sketch:

    DiagramCANVAS: Show substrate, welding torch, molten puddle of hardfacing alloy, solidified bead with carbide particles (e.g., WC) in matrix.

  • Characteristics: Thick (1-10 mm), metallurgically bonded, good for restoring worn parts (shafts, buckets).

Microstructural Treatments (Five Examples)

1. Carburizing: Diffuse carbon into low-carbon steel surface at high temp → hard, wear-resistant case (martensite) over tough core.

2. Nitriding: Diffuse nitrogen (in gas/plasma) → forms hard nitrides (ε, γ') at lower temp than carburizing → minimal distortion.

3. Induction Hardening: Rapid surface heating by induction, then quench → thin, hard martensitic layer.

4. Shot Peening: Bombard surface with small shots → induce compressive residual stresses → improve fatigue life.

5. Laser Surface Melting: High-energy laser melts thin surface layer → rapid solidification → refined microstructure, homogenized composition.

Geometrical Parameters of Coatings

• Thickness: Measured in µm. Affects load support, fatigue life.

• Roughness: Should be smooth for reducing friction/adhesive wear, or textured for oil retention.

• Porosity: Low porosity preferred for corrosion/oxidation resistance. PVD coatings typically dense.

• Adhesion: Critical for coating performance. Measured by scratch test. Depends on substrate preparation.

• Hardness Gradient: Ideally, gradual decrease from coating to substrate to avoid sharp interface stresses.

Coatings for Specific Environments

• High-Temperature Oxidation/Corrosion:

  • MCrAlY (M=Ni, Co) overlay coatings (PVD/ CVD/ spray) → form protective Al₂O₃ scale.

  • Thermal Barrier Coatings (TBCs): YSZ (Yttria-Stabilized Zirconia) by plasma spray → low thermal conductivity.

• Acidic Environments:

  • PTFE (PVD) or fluoropolymer coatings → chemically inert.

  • Ceramic coatings (Al₂O₃, Cr₂O₃) via thermal spray → excellent chemical resistance.

  • Electroplated Nickel-Phosphorus (high P content) → amorphous, corrosion-resistant.

VIII. INTEGRATED APPLICATIONS & MEASUREMENT

Friction & Wear in Engineering Systems

• Link to Failure: Excessive wear → dimensional failure, seizure. High friction → energy loss, overheating.

• Maintenance Strategy: Predictive maintenance using vibration, oil debris analysis (wear particles), thermography to monitor tribological condition.

• Design for Tribology: Select material pairs, specify surface finish, design for adequate lubrication (oil grooves, clearances), use coatings.

Tribological Testing & Evaluation

• Purpose: Compare materials/lubricants, simulate service conditions, develop wear models.

• Standard Tests:

  • Pin-on-Disc: Sliding wear. Pin (material) vs rotating disc. Measures wear volume, friction coefficient.

  • Four-Ball Tester: For lubricant EP (Extreme Pressure) properties. Measures weld load, scuff load.

  • Block-on-Ring: Simulates conformal contact (e.g., piston ring/cylinder liner).

• Interpretation: Wear rate (mm³/N·m) and friction coefficient are key outputs. Use Scanning Electron Microscopy (SEM) to analyze wear mechanisms.

[!TIP] Exam Question: "Explain any one standard friction measurement method." → Describe Pin-on-Disc: setup, procedure (load, speed, distance), measurements (friction force, wear scar), applications.

END OF UNIT 4 NOTES